fs: btrfs: Add missing cache aligned allocation

The superblock buffer must be cache aligned, since it might be used
in DMA context, allocate it using ALLOC_CACHE_ALIGN_BUFFER() just
like it was done in btrfs_read_superblock() and read_tree_node().

This fixes this output on boot and non-working btrfs on iMX53:
CACHE: Misaligned operation at range [ced299d0, ced2a9d0]

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Behún <marek.behun@nic.cz>
Cc: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>